1 Answer
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What to comply with

There are two reasons for the description of the invention in anything else than claims:

enablement requirement

written description requirement

Enablement means

The enablement requirement refers to the requirement of 35 U.S.C. 112(a) or pre-AIA 35 U.S.C. 112, first paragraph that the specification describe how to make and how to use the invention. The invention that one skilled in the art must be enabled to make and use is that defined by the claim(s) of the particular application or patent.

The purpose of the requirement that the specification describe the invention in such terms that one skilled in the art can make and use the claimed invention is to ensure that the invention is communicated to the interested public in a meaningful way.

The enablement requirement of 35 U.S.C. 112(a) or pre-AIA 35 U.S.C. 112, first paragraph, is separate and distinct from the written description requirement. Vas-Cath,Inc. v. Mahurkar, 935 F.2d 1555, 1563, 19 USPQ2d 1111, 1116-17 (Fed. Cir. 1991) (“the purpose of the ‘written description’ requirement is broader than to merely explain how to ‘make and use’”). See also MPEP § 2161. Therefore, the fact that an additional limitation to a claim may lack descriptive support in the disclosure as originally filed does not necessarily mean that the limitation is also not enabled.

(same MPEP section)

For enablement:

2164.01 Test of Enablement [R-08.2012]

Any analysis of whether a particular claim is supported by the disclosure in an application requires a determination of whether that disclosure, when filed, contained sufficient information regarding the subject matter of the claims as to enable one skilled in the pertinent art to make and use the claimed invention. The standard for determining whether the specification meets the enablement requirement was cast in the Supreme Court decision of Minerals Separation Ltd. v. Hyde, 242 U.S. 261, 270 (1916) which postured the question: is the experimentation needed to practice the invention undue or unreasonable? That standard is still the one to be applied. In re Wands, 858 F.2d 731, 737, 8 USPQ2d 1400, 1404 (Fed. Cir. 1988). Accordingly, even though the statute does not use the term “undue experimentation,” it has been interpreted to require that the claimed invention be enabled so that any person skilled in the art can make and use the invention without undue experimentation. In re Wands, 858 F.2d at 737, 8 USPQ2d at 1404 (Fed. Cir. 1988). See also United States v. Telectronics, Inc., 857 F.2d 778, 785, 8 USPQ2d 1217, 1223 (Fed. Cir. 1988) (“The test of enablement is whether one reasonably skilled in the art could make or use the invention from the disclosures in the patent coupled with information known in the art without undue experimentation.”).

(Still quoting the MPEP section on enablement, emphasis added)

So you need to include every important part of B into A to have a basis for the claims and enable them. The basis for the claims should be that every claim is literally listed and explained in the description, everything else, while theoretically possible, gets you into trouble in practice. Claims and patents are regularly invalidated because someone added something to a claim which was only between the lines or they deemed "clear from context". For enabling, references and prior art can be used too.

How to do that

If what is needed for the claims of A is in B, B has to be part of the prior art or part of the description of A. For what is part of the prior art see this answer. If B gets abandoned and not published, it won't be prior art for A, so I'd strongly advise against not making it part of A's description because it will be prior art, even if that might work. Additionally, I don't know the case law, but having to use the prior art from only one other application to enable might be undue experimentation.

So we are left with either adding every important aspect of B into A, e.g. by copying it into it, or incorporation by reference.

Incorporation by reference:

(d) “Essential material” may be incorporated by reference, but only by way of an incorporation by reference to a U.S. patent or U.S. patent application publication, which patent or patent application publication does not itself incorporate such essential material by reference. “Essential material” is material that is necessary to:

(1) Provide a written description of the claimed invention, and of the manner and process of making and using it, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains, or with which it is most nearly connected, to make and use the same, and set forth the best mode contemplated by the inventor of carrying out the invention as required by 35 U.S.C. 112(a) ;

(2) Describe the claimed invention in terms that particularly point out and distinctly claim the invention as required by 35 U.S.C. 112(b) ; or

(3) Describe the structure, material, or acts that correspond to a claimed means or step for performing a specified function as required by 35 U.S.C. 112(f) .

So an incorporation by reference only cannot be done as B is not published nor granted. Which leaves us with 37 CFR 1.57 (a) and (b) - claim priority.

If you can validly claim priority of B, you can incorporate it by reference. It won't give you an earlier filing date for the claims if they aren't based on only B, but it can allow you the incorporation by reference. Please note that I'm not citing 37 CFR 1.57 (a) and (b) which have quite some requirements to be fulfilled. Also you would have to fulfill all requirements to validly claim priority.

If you do not want to do that, the only way left is to copy every important part into the description. Which I would advise you do anyways because it gives you more possibilites to amend your claims in case they aren't granted on the first try if you incorporate it not only by copying but also logically.